1. Field of the Invention
The present invention relates to a device for removing D.C. components from output voltages of a multi-phase inverter which produces a multi-phase A.C. voltage from a D.C. voltage.
2. Description of the Related Art
FIG. 4 is a block diagram of a known single-phase inverter conventionally used as a welding power supply and of a D.C. component removing device connected to the inverter. A power rectifier 101 is connected to a three-phase power supply 102 through switches 103. A D.C. voltage derived from the power rectifier 101 is converted into an A.C. voltage V.sub.1 by an inverter 105 which has four switching elements 105a to 105d. The A.C. voltage V.sub.1 is supplied to the primary winding 107a of a transformer 107, so that a voltage is generated in the secondary winding 107b of the transformer 107. The voltage thus generated is changed again into a D.C. voltage through diodes 108a and 108b. The positive side of this D.C. voltage is connected to a welding wire 111 through a reactor 109 and a contactor 110, while the negative side of the same is connected to the welding base material 114 through a shunt resistor 113. In consequence, a welding current I is caused to flow between the wire 111 and the welding base material 114 so that the wire 111 is molten to effect welding.
During the welding, the welding current I is detected by the shunt resistor 113. The detected current component is amplified by an amplifier 116 and is supplied as a feedback signal If to a PI (Proportional-plus-Integral) operation unit 117. The operation unit 117 determines the difference between the feedback signal If and a reference value Is which is beforehand set in the welding current setting device 118. The PI operation unit 117 then conducts proportional and integration arithmetic operation so as to produce an operation signal .DELTA.e which is supplied to an adding point 138.
On the other hand, the positive and negative components of the current I.sub.1 flowing in the primary winding 107a of the transformer 107 are detected by current detectors 131 and 132, respectively. The detection output S.sub.B from the current detector 132 is subtracted at an adding point 133 from the detection output S.sub.A of the current detector 131 so that the difference between the outputs S.sub.A and S.sub.B is determined. The thus obtained difference is integrated by an integrator 134 so that a signal S.sub.I representing the integrated value is obtained.
The signal S.sub.I representing the integrated value is supplied to one end of a switching element 135 Which takes on and off states, respectively, when the polarity of the current I.sub.1 in the winding is negative and positive. The signal S.sub.I also is supplied, through a polarity inversion amplifier 137, to one end of a switching element 136 which takes on and off states, respectively, when the current I.sub.1 is positive and negative. The switching elements 135 and 136 are connected to each other at their other ends, so that these elements in cooperation produce a signal SI.sub.a representing the integrated value which is supplied to the adding point 138.
Thus, the integrated value signal SI.sub.a is subtracted at the adding point 138 from the operation signal .DELTA.e derived from the PI operation unit 117, so that a signal Se is produced and delivered to an adding point 119. At the adding point 119, the signal Se is added to a triangular wave TW generated by a triangular wave generator 120 and the sum is supplied to a comparator 121. When a condition (TW+Se)&gt;0 is met, i.e., on condition of TW&gt;-Se, the comparator 121 supplies a signal of "L" level and delivers it to a pulse distributor 122. The pulse generator 122 then conducts an operation to alternately turn on and off the switching elements 105a, 105d or 105b, 105c in the inverter 105, thereby to form the output voltage V.sub.1.
The operation of the conventional device will be described with reference to waveform chart shown in FIG. 5.
It is assumed here that there is no offset in the output voltage V.sub.1 of the inverter 105. In this case, the waveform of the voltage V.sub.1 has such a waveform that the positive and negative pulses have an equal pulse width, as shown by a broken line, so that these pulse negate each other. In consequence, the integrated value SI and the integrated value signal SIa are zero at the end of each period, as shown by broken lines. In consequence, the signal Se substantially conforms with the operation signal .DELTA.e so that a balance is maintained between the positive and negative components.
A description will be given of the case where the output voltage V.sub.1 of the inverter 105 has such a waveform that, as shown by a solid line, the widths of the positive and negative pulses are widened and narrowed, respectively, as compared with those of the waveform shown by the broken line. In this case, the value obtained through integration of the positive pulses is greater than that obtained through integration of the negative pulses, so that the integrated value SI progressively increases in the negative direction. This integrated value SI is oscillated to positive and negative sides by the operation of the switching elements 135 and 136, as well as the polarity inversion amplifier 137, whereby the integrated signal SIa is formed. Then, the signal Se is obtained as a result of subtraction of the integrated signal SIa from the operation signal .DELTA.e. The pulse distributor 122 then delivers a signal SG when the triangular wave TW is greater than -Se, i.e., when the condition TW&gt;-Se is met. Then, the inverter switching elements 105a and 105d or 105b and 105c are alternately turned on and off to produce a voltage V.sub.1.
When the voltage VI is positive, the signal -Se is greater than the operation signal -.DELTA.e, whereas, when the voltage V.sub.1 is negative, the signal -Se is smaller than the signal -.DELTA.e, so that a control is effected in such a manner as to narrow the positive pulses while widening the negative pulses. Thus, the operation signal .DELTA.e and the integrated value signal SIa are superposed on each other to enable a control for attaining a balance between the positive and negative pulses thereby nullifying the integrated value signal SIa. In consequence, it is ensured that the positive and negative pulses always have an equal pulse width, so that the transformer 107 can operate satisfactorily without any magnetic offset.
Thus, the conventional device for removing D.C. component is intended only for single-phase inverters and are not designed to adapt to a multi-phase inverter.
Namely, in case of a multi-phase inverter, e.g., 3-phase inverter, it is necessary to correct unbalance of D.C. components of all the three phases.
It is also necessary that the control is conducted such that the sum of the currents in the three phases is zero.
In consequence, quite a complicated device is required for removing D.C. components from multi-phase output of a multi-phase inverter.
Furthermore, a problem due to mutual interference between a plurality of phases also is encountered.